EUGENE WASHBURN ROARK UNIVERSITY 01 

L. R. Jones and G. W. Keitt pH, [), THESI 

Eugene Washburn Roark was born at Lexington, Ky., Sept. 5, 1894, 
and died at Minneapolis, Minn., Oct. 14, 1918. He spent his early 
youth at Lexington, where his father occupied the chair of education 
in the University of Kentucky. He received his early education in the 
schools of Lexington and Richmond, from which he went to Clark Col- 
lege, where he was granted the degree of Bachelor of Arts in 1914. 
Throughout his undergraduate training he continued to develop the 
naturalistic bent of his earlier youth and turned to the biological courses 
in anticipation of later specializing in plant pathology. In 1914 he 
entered the University of Wisconsin as a graduate student in plant 
pathology. His first research work comprised a very creditable study 
of certain aspects of the relations of Phytophthora infestans to the potato 
and the tomato. The results of these studies were presented as a thesis 
for the degree of Master of Science in 1915. From 1915 to 1917 he 
served as assistant in the Department of Plant Pathology and in 1918 
he was awarded a fellowship in this department. Throughout this 
period, he rendered faithful services of the highest quality, particularly 
in relation to the fruit disease courses and investigations of the depart- 
ment. At the same time, he prosecuted his own studies and research, 
work with vigor and success. 

The entrance of the United States into the war brought responsibil- 
ities which Roark met unflinchingly, and in the fulfillment of which he 
later gave his life. While awaiting his call to the colors, he carried the 
entire burden of the fruit disease work of the department, in the absence 
of the other members of that section of the staff, and at the same time 
completed the work for his doctorate, which was conferred in June, 1918. 
His doctor's thesis was a highly creditable dissertation on "The Sep- 
toria Leaf Spot of Rubus." On Sept. 3, before he had been able to fin- • 
ish preparing his thesis for publication, he responded to his call to the 
colors and entered the Naval Aviation Service at Minneapolis. After 
a brief period of training, he contracted influenza which rapidly developed 
into pneumonia, to which he succumbed on Oct. 14, 1918. 

Dr. Roark combined in a rare way the characteristics of the sincere 
friend, the courteous gentleman and the scholarly scientist. The keen 
sense of personal loss on the part of his university associates is tempered 
only b3^ the realization that he entered his country's service with de- 
liberation and devotion, ready to give without stint of his splendid 
young manhood, and it remains their privilege thus to continue to share 
in some small degree in his supreme sacrifice. 



Reprinted from Phytopathology, Vol. 11, August, 1921. 

THE SEPTORIA LEAF SPOT OF RUBUS 
E. W. Roark 1 

l\ 
The fungus formerly known as Septoria rubi Westendorp causes leaf 

spots on many species of Rubus (blackberries and raspberries) and at- 
tacks the steins of some of these species. The disease occurs throughout 
Europe and North America. In the United States it has been reported 
from all but seven states, and is present in all sections where the hosts 
are grown to any considerable extent. 

Wherever found, it is usually quite common but rarely serious. It 
cannot be considered a major disease of bramble fruits. The chief damage 
done is brought about by early defoliation which inhibits normal bud 
development and predisposes the canes to winter injury. 

The leaf spots vary in appearance on different hosts but at maturity 
usually show light colored centers with brownish or reddish borders. 
The pathogene also causes inconspicuous lesions on the petioles and 
canes of some hosts. 

The leaf spot fungus was named Septoria rubi by Westendorp about 
1850. This name was antedated by the possible synonyms, Ascochyta 
rubi Laschin 1832 and Ascochyta ruborum Libert in 1834, but as the 
identity of the forms thus named has not been definitely proved, it sec in;; 
advisable, for the present, to retain Westendorp as authority for the 
species name. 

The ascigerous stage, first found in 1917 in Wisconsin, is a species of 
Mycosphaerella which in its morphology does not agree with any pub- 
lished descriptions of fungi occurring on Rubus. It seems necessary, 
therefore, to describe it as a new species. 

Proof of the relation between the imperfect and the perfect form is 
based on: 1. constant association of the two in the locality where peri- 

1 When Dr. Roark responded to his call to military service, he had completed the 
preliminary draft of his doctor's dissertation and had prepared an abstract which was 
filed with the university authorities for publication in the event of his being unable 
to complete the preparation of his paper. The fuller paper was so nearly ready for 
release and was so meritorious that publication of this abstract lias been postponed in 
the hope that the original paper might yet be edited and published. This lias not been 
feasible, due to the lad that the plates and certain other important sections of the 
paper have not been found since Dr. Roark's death. It is supposed that, working 
under extreme pressure, he took some of his material with him for the final touches 
when he went into the service, and that due to his sudden and untimely death, he was 
unable to leave the necessary directions for its disposition. Under these circumstances, 
his abstract is being published, and the available parts of the original paper are being 
placed on file in the university library. — G. W. Keitt. 






\0 



*$& 



1921] Roark: Septoria Leaf Spot of Rubus 329 

thecia have boon found, 2. cellular connection of perithecia and pyc- 
nidia on the same dead loaf; 3. similar behavior of ascospores and pycno- 
spores in germination, and similarity of the two in pure culture; 4. 
positive results from inoculations with spores from the ascigerous stage. 

Pycnidia formed in spots on green leaves vary from typically well 
formed pycnidia to thin walled, imperfectly formed fruiting bodies 
which approach the acervulus in structure. Pycnidia formed on dead 
leaves are thick walled, closely grouped, and arise from heavy strom- 
ateoid masses of mycelium. It has seemed best in the discussion of 
these pycnidial forms to designate them, respectively, as summer pyc- 
nidia and winter pycnidia. 

After a study of perithecia from the different hosts the perfect stage 
is described as follows: 

Mycosphaerella rubi n. sp. Perithecia mainly hypophyllous, sometimes amphigen- 
ous, usually gregarious, erumpent, globose, 60-SOyu in transverse diameter with a short 
papilliform ostiole, black, walls pseudoparenchymatous, two or three cell layers thick, 
aparaphysate; asci subclavate to cylindrical, eight-spored, very short pedicellate, 42- 
45 by S-10/u in water; ascospores hyaline, slenderly fusiform, of two equal cells, straight 
or slightly curved, very slightly constricted at septum, 20-25 by 3.50-4.25//, extreme 
limits in length 17-28/x, sometimes tending to occur in fours in the ascus, usually ir- 
regularly biseriate. Conidial stage: Septoria rubi Westendorp. Hab. on fallen leaves 
of Ruhus strigosus, R. parviflorus, R. allegheniensis and R. hispidus in Door County, 
Wisconsin. 

In studies of the germination of pycnospores and ascospores it was 
found that they readily germinate in water and nutrient solutions and 
that pycnospores reflect strain differences in their variable behavior in 
germination. Some pycnospores and ascospores have shown a definite 
reaction to strong, diffused sunlight, the developing germ tubes exhib- 
iting negative heliotropism. 

The minimal temperature for germination in water or in a favorable 
nutritive medium is slightly below 2° C; the optimal, between 18° and 
26°, about 23°; and the maximal, about 32° or slightly above. 

About fifty strains of the fungus have been carried in pure culture, 
isolations having been made at different times of the year from the 
leaves and canes of various host species. Strains from the same or 
different hosts varied considerably in regard to type and amount of spore 
production, and in stromatic development, some readily forming pyc- 
nidia, while others formed only masses of needle-shaped secondary 
conidia. When grown on about thirty media, the fungus showed only 
minor variations, except that Lima bean agar was found to favor the 
production of secondary conidia even in case the fungus was usually 
stromatic. 



330 Phytopathology [Vol. 11 

The minimal temperature for growth on a nutrient substratum, 
potato agar, is less than 2°C, the optimal, between 20° and 23°; and 
the maximal, about 32°. 

It is believed that the production of secondary conidia is primarily 
a strain characteristic varying greatly with different strains, but that 
within a given strain it can be encouraged to a certain extent by crowd- 
ing of spores at the time of germination, frequent transferring, use of 
special media, and optimal temperature for growth. 

The stromateoid type of pure-culture growth will live for several 
months at room temperature (about 22° to 25°C); but, at the same 
temperature, cultures consisting of masses of secondary conidia lose 
their viability within about two weeks. 

The results of many inoculations with spores derived from both 
pycnidia and perithecia may be briefly stated as follows: 1. spores 
from the ascigerous stage were found to produce leaf infection, resulting 
in lesions which sometimes contained typical pycnidia but usually im- 
perfectly formed fruiting bodies; 2. the stem form of the fungus was 
proved to be identical with the leaf form; 3. blackberry strains would 
not cross to raspberries, nor would the raspberry strains infect black- 
berries. Conclusions were drawn from no inoculations except those 
which were adequately controlled by uninoculated plants and from 
which successful reisolations were made. 

The fungus commonly overwinters as mycelium and immature pyc- 
nidia in dead leaves and, in the case of red raspberries, in the bark on 
canes. Perithecia arc a factor in the overwintering, but are restricted in 
occurrence. 

In Wisconsin, leaf spots usually begin to appear two or three weeks 
after the leaves are well opened and continue to increase in number 
until frost, the amount, of infection depending primarily upon weather 
conditions. 

Primary infection is brought about mainly by newly formed pyeno- 
spores from overwintering pycnidia on dead leaves or bark, though 
ascospores may also function in this way. Secondary infection through- 
out the season is caused by pyenospores from the current-season lesions. 

Mature ascospores can be found late in May, and continue to develop 
well into July — occurring most abundantly during June. Pycnidia are 
formed in practically all lesions, their frequency depending somewhat 
upon the host variety. Viable pyenospores can be found throughout 
the year but are more abundant at certain times. This is due to the 
fa< I Unit there are usually so-called waves of infection which are corre- 
lated with periods of rainfall. 



1921] Roark: Septoria Leaf Spot of Rubus 331 

The period of incubation for the fungus in leaves was found to vary 
from 8 to 11 days in most inoculation experiments, although wider 
variations sometimes occurred. From observations it would seem that 
in nature the incubation period may show even greater variations. 

Field infection is favored by moderately cool weather and frequent 
periods of rainfall. 

As a result of observations and experiments, it is believed that the 
disease is spread into cultivated plantations primarily by diseased nur- 
sery stock, the fungus being carried in the bark or persistent leaves. 
The principal agents of spore dissemination are wind in case of the 
ascospores, which are forcibly discharged into the air, and splashing or 
wind-borne rain in case of pyenospores, which exude in masses and can 
be scattered only after being separated in water. 

Among the host species and varieties observed by the writer, the dew- 
berries and smooth-leafed blackberries have shown the greatest amount 
of spotting, while certain black raspberries and R. odoratus have been 
least affected. These differences have not been explained. 

The following tentative suggestions for controlling the disease are 
based primarily upon the work of others, coupled with the writer's 
observational evidence. 

1. Care should be taken to obtain disease-free plants for setting out 
new plantations. 

2. Sources of inoculum should be reduced by destruction of dead 
leaves and old canes in the fall. 

3. When conditions for the development of leaf spot are favorable, 
pray with Bordeaux mixture, 3-3-50, after the leaf buds are well opened, 
nd at intervals of two or three weeks until the fruit is two-thirds grown. 

LITERATURE 

This list of references was used by Dr. Roark in preparing tlie more extended paper 
referred to on page 328. 
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332 Phytopathology [Vol. 11 

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1921] Hoark: Septoria Leaf Spot of Rubus 333 

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